The present invention relates generally to the art of locating a blood vessel lesion in a human subject, and more particularly, to an apparatus and method to efficiently identify a lesion over an entire patient""s peripheral arterial vasculature and grade any identified stenosis using magnetic resonance imaging (MRI) technology.
Arteries are the blood vessels emanating from the heart that supply the necessary nutrients to the organs and tissues of the human body. A narrowing or constriction of an artery reduces the delivery of nutrients, such as oxygen to the recipient tissue and has profound effects on tissue function. In general, significant narrowing of an artery leads to compromised function of the organ in question, at best, and organ failure or death at worst. Stenosis or narrowing at any number of locations along the course of the arteries from the abdominal aorta through the calf can result in compromise of arterial blood flow to the distal lower extremities. The evaluation of the peripheral vessels is further complicated by the high incidence of tandem or synchronous lesions, any one of which could be the underlying cause for diminished arterial blood flow. Furthermore, the surgical decisions for potential bypass procedures to improve distal blood flow are greatly affected by the ability to assess the arteries in the foot. As a result, the successful imaging of the lower extremities (i.e. the peripheral run-off study) requires not only the accurate assessment of the presence and functional significance of a narrowing, but also the ability to evaluate the entire arterial course of the peripheral arterial tree from abdominal aorta to the foot. It is known that the flow in the vessel at the point of narrowing and immediately after the narrowing is characterized by rapid flow velocities and/or complex flow patterns. Quantitative flow data can readily aid in the diagnosis and management of patients and also help in the basic understanding of disease processes.
There are many techniques available for the assessment of the peripheral arteries that include traditional invasive catheter angiography and ultrasound. Because conventional x-ray angiography requires catheterization and the use of nephrotoxic iodinated contrast agents, it is reserved as the final option. Screening for peripheral arterial occlusive disease (PAOD) is typically performed using non-invasive methods such as ultrasound or plethysmography. However, neither of these techniques can provide angiographic illustration of the vessels and merely provides the assessment of individual segments of the intervening arterial anatomy. Both techniques are operator dependent and have confounding technical difficulties which make the imaging often tedious to perform. Moreover, neither technique can provide the comprehensive information required for surgical planning and traditional x-ray angiographic depiction is generally required as an adjunct for pre-operative management.
Magnetic resonance angiography (MRA) is an emerging method for the non-invasive assessment of arteries. Up to now, the application of MRA has been tailored to individual smaller vascular territories (40-50 cm fields of views). With the ability now to translate the table and imaging multiple overlapping fields-of-view, MRA can now be prescribed to image a much larger area such as necessary for evaluation of PAOD. The use of intravenously administered contrast agents for contrast-enhanced MRA, in particular, has enabled the depiction of 1-1.2 meters of arterial anatomy in less than 1 minute. MRA can also be performed using a number of methods. One technique, phase contrast (PC) MRA is a practical and clinically applicable technique for imaging blood flow. MRI utilizes radio frequency pulses and magnetic field gradients applied to a subject in a strong magnetic field to produce viewable images. When a substance containing nuclei with net nuclear magnetic moment, such as the protons in human tissue, is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field (assumed to be in the z-direction), but precess about the direction of this magnetic field at a characteristic frequency known as the Larmor frequency. If the substance, or tissue, is subjected to a time-varying magnetic field (excitation field B1) applied at a frequency equal to the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be nutated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated (as the excited spins decays to the ground state) and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Phase contrast MRA makes use of flow encoding gradient pulses which impart a velocity-dependent phase shift to the transverse magnetization of moving spins while leaving stationary spins unaffected (Moran P. R. A Flow Velocity Zeugmatographic Interlace for NMR Imaging in Humans. Magnetic Resonance Imaging 1982; 1: 197-203). Each phase contrast acquisition generates two images: a magnitude image that is proportional to the proton density of the object and may also be T1-weighted, and an image representing the phase of the object. The phase image produced has information only from the moving spins and the signal from stationary tissue is suppressed. Images representing both the average flow velocity over the entire cardiac cycle and at a series of individual points in the cycle have been generated using this technique. The phase contrast MR method produces phase images with intensities that represent the magnitude of the flow velocity and also the direction of flow. Therefore, such images may be used for both qualitative observation of blood flow and quantitative measurement. The practical application of phase contrast MR angiography and venography to the quantitative determination of flow velocity is therefore evident.
It would be advantageous to use magnetic resonance imaging technology to efficiently locate and identify a stenosis in a blood vessel along a patient""s peripheral arterial vasculature and use this MR technology to grade the stenosis for follow up care. It would also be advantageous to use a contrast agent bolus injection to increase the image signal-to-noise ratio in the arterial vessels during the first passage of the contrast material to enhance the screening technique. However, to do so, a multi-station acquisition sequence must be used to scan the entire peripheral vasculature as the contrast bolus travels through the body. Previous attempts at using MR technology to improve the ability to detect and grade peripheral arterial stenoses have relied primarily on using a single anatomic scan to visualize the location of a stenotic vessel segment. In this method, it was desirable to achieve the highest spatial resolution possible by decreasing pixel size. In addition, in order to minimize flow-related artifacts such as intravoxel dephasing that can overestimate the degree of stenosis, the prior art employed first moment gradient nulling for flow compensation and short echo time (TE) parameters.
It would be desirable to improve on this prior art by accomplishing the converse. That is, to utilize the presence of flow-related artifacts to improve the detection of an arterial stenosis by sensitizing the image acquisition to intra-voxel flow dephasing effects, thereby exacerbating flow voids and increasing the conspicuity of arterial lesions in a quick screening scan. It would also be advantageous to have a method and apparatus for efficient visualization of a stenosis (i.e. lesions or narrowings) using MR technology for screening patients, followed with a more thorough and lengthy exam of the individual stenoses, enabling a more time-efficient examination. It would also be advantageous to use a contrast agent bolus injection to increase the image signal-to-noise ratio in the arterial vessels during the first passage of the contrast material to enhance the screening technique. However, to do so a multi-station acquisition sequence must be used to scan the entire peripheral vasculature as the contrast bolus travels through the body.
The present invention relates to a method and apparatus for efficient stenosis identification in peripheral arterial vasculature using MR technology, that solves the aforementioned problems.
The present invention includes a two step approach to accurately identify any blood vessel lesions and then if a lesion is found, specify the degree of stenosis. In the initial step, an examination for lesion identification is disclosed that includes tracking a contrast bolus as it passes through the arterial vasculature of a patient and acquiring a series of low spatial resolution MR images as the contrast bolus travels through the patient""s vasculature. Preferably, the MR image is acquired using a gradient echo imaging pulse sequence with a flow sensitive bi-polar gradient waveform. The bi-polar gradients generate a broad distribution of velocities in a large voxel. Since a stenosis present in a given voxel will result in intra-voxel flow dephasing in voxels immediate to and distal to the stenosis, a stenosis can be quickly and efficiently localized using the initial step. After a stenosis is identified, a second step is performed in which a high spatial resolution MR image is acquired for more accurate and specific grading of the stenosis in a targeted area.
According to one aspect of the invention, a method of identifying a stenotic vessel in a patient""s peripheral arterial vasculature using MR imaging is disclosed which includes tracking passage of a contrast bolus through the patient and simultaneously performing a screening study by acquiring a series of first, fast MR images having a low spatial resolution along the patient""s peripheral arterial vasculature as the contrast bolus passes through the patient to scan for a suspected stenosis. The method next includes scanning the series of first MR images to identify a suspected stenosis, then performing a detailed study by acquiring a second MR image having a higher resolution than the series of first MR images for grading the identified stenosis.
In accordance with another aspect of the invention, an examination method is disclosed to identify a lesion in a blood vessel of a patient""s peripheral arterial vasculature and grade a stenosis resulting therefrom. The examination includes first scanning the peripheral vessels, for example using contrast-enhanced MRA based on gradient echo imaging pulse sequence having a flow sensitizing bi-polar gradient waveform across a patient""s peripheral arterial vasculature, and then detecting and localizing a suspected stenosis using the series of first MR images. The method next involves acquiring a second MR image if a stenosis is detected and localized. The second MR image has a higher resolution than the series of first MR images and is acquired in a region in which the suspected stenosis is detected and localized to grade the suspected stenosis. If a stenosis is not detected and localized, the examination is ended without further MR image acquisitions.
In accordance with another aspect of the invention, an MRI apparatus is disclosed to conduct MR stenosis screening, and if necessary, grade a stenotic vessel that includes an MRI system having a number of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field, an RF transceiver system, and an RF modulator controlled by a pulse control module to transmit RF signals to an RF coil assembly in order to acquire MR images. The MRI apparatus also includes a computer programmed to operate the MRI system in two modes of operation to efficiently conduct a stenosis exam across an entire patient""s peripheral arterial vasculature. The first mode is programmed to acquire a series of first MR images with low resolution over the patient""s peripheral arterial vasculature, then receive an input to either end the stenosis exam or switch to a second mode of operation if a stenosis is indicated in the series of first MR images. In the second mode of operation, the computer is programmed to localize a field-of-view (FOV) to target the stenosis, and then acquire at least one second MR image with resolution higher than that of the series of first MR images of the localized FOV.
In accordance with yet another aspect of the invention, the aforementioned methods are implemented in a computer program that is fixed on a computer readable storage medium that, when executed, causes the computer to acquire a series of first MR images of a patient""s peripheral arterial vasculature. Each of the first MR images in the series of first MR images is acquired within a scan station as a contrast bolus travels therethrough. The series of first MR images has high phase cancellation to screen a patient for possible arterial lesions. The computer is further programmed to limit a FOV to a target region within the patient""s peripheral arterial vasculature if a lesion is located, and then acquire a second MR image of the targeted region. The second MR image having a resolution higher than that of the series of first MR images, and only being acquired if the series of first MR images indicates the presence of a lesion, or stenosis.
In this manner, the higher resolution targeted acquisition near the site of interest is performed only if a lesion is present to effectively grade the stenosis. This technique provides a two-step technique involving a first step with increased sensitivity to detect lesions that can be acquired quickly, across the entire peripheral arterial vasculature and then only performing the more time-consuming second step of acquiring an image with high specificity for grading the lesion only if one is detected in the first step. This two-tiered approach increases the efficiency for accurate peripheral vasculature stenosis detection and assessment.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.